In recent years, a Long Term Evolution (LTE) service which is standardized in the 3rd generation partnership project (3GPP) has been provided.
As illustrated in FIG. 1, a wireless communication system based on the LTE system includes, for example, a plurality of wireless base stations 200-1 and 200-2 and at least one wireless terminal 400.
In a wireless communication system 100, when the wireless terminal 400 moves from a wireless area 300-1 to a wireless area 300-2, a handover is performed. In addition, the concept of the wireless areas 300-1 and 300-2 includes, for example, cells or sectors provided by the wireless base stations 200-1 and 200-2. Hereinafter, in some cases, for convenience of explanation, the wireless area is simply referred to as a cell.
The wireless terminal 400 measures the received power or the reception quality of radio signals transmitted from the wireless base station 200-1 which provides the cell 300-1 and the received power or the reception quality of radio signals transmitted from the wireless base station 200-2 which provides the cell 300-2. Then, the wireless terminal 400 transmits a report (MR: measurement report) including the measurement result to the wireless base station 200-1 which is a handover source.
When receiving the MR, the wireless base station 200-1 may start a handover process. For example, when receiving the MR from the wireless terminal 400, the wireless base station 200-1 starts the sequence of the handover process and determine a cell, which is a handover destination, on the basis of various kinds of information included in the received MR.
In the example illustrated in FIG. 1, a handover from the cell 300-1 as a serving cell to the cell (adjacent cell) 300-2 adjacent to the cell 300-1 is determined. That is, in the example illustrated in FIG. 1, the adjacent cell 300-2 is determined as a handover target cell.
After the handover destination is determined, the wireless base station 200-1, which is a handover source, transmits, to the wireless terminal 400, a handover instruction message including various kinds of information about the target cell 300-2 or the wireless base station 200-2 which is a handover destination.
The wireless terminal 400 which receives the message from the wireless base station 200-1 is handed over from the serving cell 300-1 to the target cell 300-2 and is connected to the wireless base station 200-2.
On the other hand, at least one of the conditions that the wireless terminal 400 measures the received power or the reception quality and the conditions that the wireless terminal 400 transmits the MR to the wireless base station 200-1 are set (transmitted) from the wireless base station 200-1 to the wireless terminal 400 in advance.
For example, the wireless terminal 400 regularly or irregularly measures the received power or the reception quality of the cell 300-1 and the received power or the reception quality of the adjacent cell 300-2. When the transmission conditions of the MR which are transmitted from the wireless base station 200-1 in advance are satisfied on the basis of the measurement result, the wireless terminal 400 transmits the MR to the wireless base station 200-1 which provides the serving cell 300-1. In addition, for example, the MR may include other information items related to the adjacent cell 300-2 in which the transmission conditions of the MR are satisfied.
Here, Event A3 which is one of the transmission conditions of the MR in the LTE system is represented by Expression (1).[Expression 1]Mn>Ms+MRmargin  (1)
In Expression (1), Mn is the received power [dBm] or the reception quality [dB] of an adjacent cell and Ms is the received power [dBm] or the reception quality [dB] of a serving cell. In addition, MRmargin is a margin value [dB] between Mn and Ms.
When the measurement results of Ms and Mn satisfy the transmission conditions [see Expression (1)] of the MRfor, for example, the time defined by TTT (Time To Trigger), the wireless terminal 400 transmits the MR to the wireless base station 200-1 to which the wireless terminal 400 is being connected and by which the handover process for the wireless terminal 400 is executed. In addition, TTT is a parameter indicating the stability of the received power or the reception quality with respect to time.
In 3GPP, for LTE, Self-Organizing Networks (SON) have been examined which automatically set various wireless parameters to perform optimization, thereby improving the efficiency of the management operation of the operator (administrator) (see 3GPP TR36.902 v9.2.0).
Among the SON use cases, Mobility Load Balancing (MLB) is considered which adjusts the parameters included in the transmission conditions of the MR between adjacent cells to balance a load, such as CPU utilization or wireless resource utilization, in each wireless base station.
In the wireless communication system using MLB, for example, when the load of the wireless base station 200-1 is equal to or greater than a certain threshold value, the transmission conditions of the MR may be changed (updated) such that the value of MRmargin in Expression (1) is reduced.
In this case, as illustrated in FIG. 2, the position of the wireless terminal 400 at the time when the MR is transmitted after MRmargin is reduced is closer to the wireless base station 200-1 than the position of the wireless terminal 400 at the time when the MR is transmitted before MRmargin is reduced.
In the example illustrated in FIG. 2, when receiving the MR from the wireless terminal 400, the wireless base station 200-1 may start the handover process using the adjacent cell 300-2 as a target cell.
That is, when the value of MRmargin is reduced, the start position of the handover for the wireless terminal 400 from the cell 300-1 to the cell 300-2 may shift closer to the wireless base station 200-1 than the start position of the handover of the wireless terminal 400 from the cell 300-1 to the cell 300-2 before MRmargin is reduced.
Therefore, it is easy to handover the wireless terminal 400 from the cell 300-1 provided by the wireless base station 200-1 to the cell 300-2 provided by the wireless base station 200-2. As a result, it is possible to reduce the load of the wireless base station 200-1.
Meanwhile, for example, when the load of the wireless base station 200-2 is equal to or greater than a certain threshold value, the transmission conditions of the MR may be changed (updated) such that the value of MRmargin included in the MR increases.
In this case, as illustrated in FIG. 3, the position of the wireless terminal 400 at the time when the MR is transmitted after MRmargin increases is closer to the wireless base station 200-2 than the position of the wireless terminal 400 at the time when the MR is transmitted before MRmargin increases.
Therefore, it is hard to handover the wireless terminal 400 from the cell 300-1 provided by the wireless base station 200-1 to the cell 300-2 provided by the wireless base station 200-2. As a result, it is possible to reduce the load of the wireless base station 200-2.
Japanese Laid-open Patent Publication No. 2009-147920 discloses a method in which a mobile station transmits a measurement report (MR) on the basis of a measurement rule indicating whether to transmit the MR which is transmitted from a wireless base station.
When the wireless terminal 400 transmits the MR, it consumes resources, such as transmission power or a wireless communication bandwidth. Therefore, as the transmission frequency of the MR increases, the consumption of the resources increases.